In digital radio communications, expectations have been placed on adaptive array antenna (hereinafter referred to as “AAA”) technology, whereby weighted addition is performed of antenna element outputs from a plurality of antenna elements using a complex coefficient (hereinafter referred to as “weight”), as a technology for improving communication quality. With this AAA technology, interference waves can be suppressed by controlling directivity adaptively, making use of the fact that the direction of arrival differs for transmission waves from a plurality of communicating parties. AAA technology is therefore suitable as a method of canceling interference waves from other channels.
This AAA technology is described in detail in, for example, NTT DoCoMo Technical Journal Vol. 5 No. 4, P25-P36.
With AAA technology, waves arriving from various directions are received by a plurality of antenna elements, and a final reception signal is obtained by a process of combination using an appropriate weight on each reception signal according to its direction of arrival. Therefore, reception quality is greatly affected by whether or not the weights are appropriate.
For this reason, various weight control algorithms for finding appropriate weights have been devised in AAA technology. A typical algorithm uses a desired signal waveform as prior knowledge, and also uses the root-mean-square value of the error of antenna element outputs with respect to a reference signal (desired signal) as an output evaluation function.
In this case, an LMS (Least Mean Square) or RLS (Recursive Least Squares) algorithm commonly used by a conventional adaptive equalizer can be employed.
However, in mobile communications, transmission path characteristics fluctuate at high speed since terminals are moving, and there are consequently problems with trackability in conventional AAA technology. That is to say, it is necessary to track the direction of arrival of transmission waves fluctuating at high speed, and find appropriate weights successively in a short time.
Thus, with a weight control algorithm, if the speed of changes in the direction of arrival of a transmission wave due to movement of a mobile terminal exceeds the optimal weight calculation time, reception quality will degrade. There is consequently a need for an apparatus and method that will enable optimal weights for an adaptive array antenna to be calculated even a little faster.